Ropes and harnesses play an important role in personal protection for use in areas such as rock climbing, parachuting and in industrial applications. These supports are designed to operate under two forms of loading: normal static loading in which they are exposed to operational design loads; and shock loads which occur, for example, after a fall.
Ropes for use in these applications are typically classified as possessing certain properties, such as maximum safe loading, elongation under load, and shock load strength. Under normal usage many ropes will retain these properties for a considerable time. However, when subject to loads above the yield point of the rope, damage may occur which would seriously reduce the performance of the rope in further safety critical events.
Unfortunately, the occurrence of such damage to a rope may not be readily apparent from a visual inspection of the rope. On the one hand, this may result in the continued use of a damaged rope if, for example, a fall on a rope is not reported or recorded. On the other hand, ropes may be discarded as a cautionary measure after a minor fall which has not actually resulted in any damage to the rope.
Damage may also occur to ropes under less dramatic circumstances, such as if the rope is bent over a small radius and loaded.
Various means have been proposed for providing an indication of damage to ropes and webs. In U.K. Patent No. 1,413,200 to Takata Kojyo Co., Ltd., there is disclosed a strain indicating web for use in a safety belt. Indicator threads of different colour from the web are provided and have an elongation to rupture below that of the ground warps of the web, such that elongation of the web beyond a predetermined value results in rupture of the indicator threads. In U.K. Patent Application No. 2,130,611A to Thurston, a synthetic fibre rope is disclosed which includes a number of monitoring optical plastic fibres. The fibres are selected such that when subjected to the nigh local strains associated with damage to the rope their optical conductivity is destroyed or reduced, to provide a warning of impairment. U.K. Patent Application No. 2152088A to Bridon plc (United Kingdom) discloses a fibre rope incorporating at least one electrical, optical, of fluid transmission element, such as a carbon fibre yarn whose mechanical properties approximate to those of the rope. Failure of the rope can be detected by testing whether the transmission element transmits a signal or matter. In U.K. Patent No. 2194256B to Hutchings, webbing strap or rope is disclosed which incorporates material which is subject to stress-whitening under predetermined loads. The stress-whitening material may be provided in the form of a patch, label or sleeve or may be in the form of one or more strands or threads pleated, woven or braided into the rope.
All of the disclosures contained in the above noted patent documents require the incorporation of separate strands or fibres into the rope or web, which complicates the manufacture of the rope and possibly affects its properties. The Hutchings patent discloses the use of patches, labels and sleeves which could be separately fixed to the web or rope after manufacture but would only provide an indication of damage to the rope at the site of the patch, label or sleeve.
Further, these various proposals rely on the behaviour of a strain indicating thread, fibre or element, rather than the actual load bearing fibres of the rope or belt and it may be difficult to ensure that the strain indicating element is subject to the same load conditions as the rope.
It is one of the objects of the present invention to provide a rope, web or the like which obviates and mitigates these disadvantages.